1. Field of the Invention
The present invention relates to an antenna device and a radio communication terminal that receive radio wave signals.
2. Description of the Related Art
In recent years, diversity reception is employed in radio communication terminals including cellular phones. In the diversity reception, a transmitter-receiver antenna element and a receive-only antenna element are connected to a reception device, and signals received by the antenna elements are selected in accordance with the reception electric field intensity, so that, of the signals received by the antenna elements, a signal whose reception electric field intensity is the higher is selected. Or, the signals received by the respective antenna elements are combined.
On the other hand, further downsizing and high-functionalization of radio communication terminals is desired. Thus, it becomes an absolute must, to make the radio communication terminals have antenna elements built-in, while the parts-mounting area becoming smaller.
Accompanied by the downsizing and high-functionalization of radio communication terminals, the downsizing of antenna elements is proceeding.
When the above-mentioned transmitter-receiver antenna element and receive-only antenna element are built in a radio communication terminal, the antenna performance, such as antenna gain and reception sensitivity, tends to be deteriorated due to some circumstances, such as closeness to the peripheral electronic parts and ground (GND) area, and so forth. Further, with the secured areas being insufficient for installing both the antenna elements to be adequately apart from each other, the transmitter-receiver antenna element and the receive-only antenna element are installed to be closer than is necessitated. Such closer installation causes interference between the antenna elements, and the antenna performance thus tends to be deteriorated. Furthermore, there also is a problem that an antenna installed to a radio communication terminal has a narrower receivable frequency bandwidth since such antenna is small.
Accordingly, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2000-31721 discloses a cellular phone in which an electromagnetic shielding is provided between two helical coil elements constituting a diversity antenna and perpendicularly provided on the upper portion of the casing of the cellular phone. In the disclosed structure, this electromagnetic shielding suppresses the mutual interference less and improves the diversity effect. Further, Unexamined Japanese Patent Application KOKAI Publication No. 2000-31721 discloses a method for achieving an electromagnetic coupling in a cellular phone of such structure, in which an additional element, such as a parasitic helical coil element, is provided besides the two helical coil elements, in order to be compatible with bandwidth-widening of frequencies for transmission and reception, or be compatible with multiband, which involves widely separated plural frequencies.
Further, in a portable radio communication terminal disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2004-72605, a first antenna element is built in a first casing, and a second antenna element is built in a second casing that is foldable over the first casing. Then, when the first casing and the second casing are in the opened position, the first antenna element and the second antenna element serve as a diversity antenna, as constituents thereof. Further, when the first casing and the second casing are in a closed position, the second antenna element serves as a parasitic element such that it operates by electromagnetic coupling with the first antenna element, thereby widening the bandwidth of the first antenna element.
However, the structure disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2000-31721 provides additional elements such as an electromagnetic shielding and a parasitic helical coil element, besides two helical coil elements that constitute a diversity antenna. For this reason, there is a problem that the structure of the antenna device becomes complex.
Further, the structure disclosed in Unexamined Japanese Patent Application KOKAI Publication No, 2004-72605 include two built-in antenna elements constituting a diversity antenna separately in the foldable two casings. Accordingly, such structure cannot be applied to a radio communication terminal constituted by a single casing, thus not being versatile.
The present invention is made in view of this, and it is an object of the present invention to provide an antenna device which can perform diversity reception and can widen a frequency bandwidth within which the main antenna can transmit and receive a radio wave, and which can be installed on both of a radio communication terminal having one casing and a radio communication terminal having two casings, and to provide a radio communication terminal equipped with the antenna device.